Key Benefits

Estimate your active (free) testosterone level from total testosterone and SHBG.

• Spot androgen excess linked to PCOS, acne, hirsutism, or scalp hair thinning.
• Provide objective evidence of hyperandrogenism to support or refine a PCOS diagnosis.
• Guide care by indicating need for lifestyle, hormonal, or anti-androgen therapies.
• Protect fertility by flagging androgen-driven ovulation issues that can delay conception.
• Track trends to monitor treatment response and long-term androgen balance.
• Flag metabolic risks often associated with hyperandrogenism, like insulin resistance.
• Best interpreted with symptoms and DHEAS; unreliable on estrogen therapy or during pregnancy.

What is Free Androgen Index (FAI)?

The Free Androgen Index (FAI) is a calculated marker that estimates how much testosterone in your blood is unbound and available to act on tissues. It relates the amount of circulating testosterone to the amount of sex hormone–binding globulin (SHBG), the liver‑made protein that tightly carries sex hormones in the bloodstream. Testosterone itself is produced mainly in the testes in men and in the ovaries and adrenal glands in women. By considering SHBG, FAI provides an indirect window on the “free” fraction of testosterone.

FAI matters because the free portion of testosterone is the part that can enter cells and activate androgen receptors, driving androgen effects on muscle, hair follicles, skin, bones, mood, and metabolism. In other words, FAI reflects the balance between hormone production and protein binding, giving a practical sense of androgen availability (bioavailable/free testosterone). It is not a hormone and not a direct measurement; it is a convenient proxy that helps summarize overall androgen status when a straightforward estimate of free testosterone is needed.

Why is Free Androgen Index (FAI) important?

Free Androgen Index (FAI) estimates how much testosterone is freely available to act on tissues by relating total testosterone to sex hormone–binding globulin (SHBG). Because androgens influence muscle, bone, red blood cell production, skin, hair, mood, metabolism, and reproductive function, FAI offers a systems-level view of androgen activity rather than a single number in isolation.

Reference intervals are sex-, age-, and lab-specific: values are naturally much lower in women than in men, rise around puberty, and fall with age. Pregnancy and higher estrogen states raise SHBG, pushing FAI lower. For most adults, “healthy” tends to sit in the middle of the sex- and age-specific range.

When FAI is lower than expected, it reflects reduced free testosterone or excess SHBG. Physiology shifts toward lower anabolic tone: fatigue, low libido, reduced muscle strength, lower bone support, and sometimes mild anemia or low mood. Men may notice fewer morning erections and decreased sexual function. In women, desire and energy can dip; in teens, puberty may progress slowly. Very low FAI in pregnancy is usually physiologic due to high SHBG.

When FAI is higher, either testosterone is elevated or SHBG is suppressed (often with insulin resistance or obesity). Androgen effects become more pronounced: oily skin, acne, and scalp hair thinning; in women, increased facial/body hair, irregular or absent ovulation, and features suggestive of polycystic ovary syndrome. Men may experience acne, elevated hematocrit, and prostate-related symptoms; adolescents may show early pubic hair and severe acne.

Big picture: FAI integrates the androgen signal with SHBG biology, linking reproductive, metabolic, liver, and thyroid systems. Interpreting it alongside symptoms and related tests (total testosterone, SHBG, gonadotropins, DHEAS, estradiol, thyroid and liver markers, glucose/lipids) helps clarify long-term risks for fertility, bone health, and cardiometabolic outcomes.

What Insights Will I Get?

Free Androgen Index (FAI) estimates how much testosterone is available to tissues by relating total testosterone to sex hormone–binding globulin (SHBG). It links gonadal output to binding capacity, so it reflects androgen “signal” at the system level. Androgens shape energy production, muscle and bone maintenance, glucose and lipid metabolism, red blood cell production, sexual function and fertility, skin and hair biology, mood, and cognition.

Low values usually reflect reduced androgen bioavailability from low testosterone, high SHBG, or both. This lowers anabolic tone and may show up as fatigue, decreased strength, reduced libido, mood changes, and lower bone turnover. In men, age-related SHBG rises commonly drive a lower FAI; in pregnancy and with oral estrogens, SHBG rises physiologically and FAI falls without implying disease. Women naturally run much lower FAI than men, so “low” is interpreted against female-specific ranges.

Being in range suggests adequate androgen signaling relative to binding proteins, supporting stable energy, body composition, sexual function, hematologic balance, and metabolic flexibility. There is no universal consensus on an “optimal” spot, but most healthy individuals fall near the mid portion of sex-specific reference intervals.

High values usually reflect greater androgen bioavailability due to high testosterone, low SHBG, or both. Low SHBG is common with insulin resistance, obesity, and hypothyroidism. In women, high FAI tracks clinical androgen excess (hirsutism, acne, ovulatory disruption, PCOS phenotypes). In men, very high values often indicate exogenous androgens or SHBG suppression and can associate with acne, oily skin, and higher hematocrit.

Notes: FAI is a surrogate and is less reliable when SHBG is very high or very low. Timing (morning sampling), acute illness, thyroid and liver status, pregnancy, menopause, puberty, and medications (estrogens, androgens, anticonvulsants) all shift SHBG and influence interpretation.